'use strict';
var single_element = function(collection){
    //1.选出第偶数个元素
    collection = collection.filter((value,index) =>{
        if(index %2 !== 0){
            return value ;
        }
    }) ;
    //2.选出不重复的元素
    let arr = [] ;
    for(let value1 of collection ){
        if(collection.indexOf(value1) === collection.lastIndexOf(value1)){
            arr.push(value1) ;
        }
    }
    return arr ;
}
module.exports = single_element;
